Spatial light modulator

ABSTRACT

A light modulator and a high speed spatial light modulator (230) with each pixel (231) made of stacked quarter wavelength layers (232, 234) of heterogeneous material. Each layer (232, 234) is composed of periodic quantum well structures whose optical constants can be strongly perturbed by bias on control electrodes (240, 242). The control electrodes (240, 242) act to either remove light absorbing electrons from the layer or to inject them into each layer. The effect is to produce either a highly relecting mirror or a highly absorbing structure. The spatial light modulator (230) is compatible with semiconductor processing technology. Also, a modulator invoking the Burstein effect in the form of a stack of p-n diodes is disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to optical devices, and, moreparticularly, to light modulators of varying carrier densities insemiconductor materials.

Highly reflective mirrors made of alternating layers of non-absorbingmaterials are well known; see Jenkins and White, Fundamentals of Optics,ch 14 (McGraw Hill 1957), and FIG. 1A for a schematic perspective viewof such a mirror and FIG. 1B for a graph of the reflectance. In suchmirrors the two layer types have different optical constants ("n" is theindex of refraction and "k" the attenuation so that ε=(n+ik)²); andbecause there is a discontinuity in the optical constants at each layerinterface, light which enters the mirror undergoes multiple reflections.If the optical thickness of each layer is chosen correctly (a quarterwavelength plus optional multiples of a half wavelength), the reflectedrays will be in phase as illustrated in FIG. 1C and the mirror will havehigh reflectivity as illustrated in FIG. 1B. Narrow band reflectivity of98% is routinely obtained in such multilayer structures.

To make such a multilayer mirror efficient, the optical absorption ofeach layer must be very small. Otherwise, significant optical absorptionwill take place within each layer and the subsequent multiplereflections within the mirror will further reduce the intensity of theinternal light rays.

If the optical constants of the layers of such a multilayer mirror couldbe switched between absorbing and non-absorbing values, then a mirror ofadjustable reflectivity (i.e. a light modulator) would result. Buttraditionally, the optical constants of a material can only be slightlyadjusted by, for example, electric fields (the Pockels effect), which isinsufficient to make a reasonable spatial light modulator. Thus it is aproblem in the known multilayer mirrors to switch the optical constantsof the layers.

Quantum well devices are known in various forms, heterostructure lasersbeing a good example. Quantum well heterostructure lasers rely on thediscrete energy levels in the quantum wells to achieve high efficiencyand typically consist of a few coupled quantum wells; see, generally,Sze, Physics of Semiconductor Devices, 729-730 (Wiley Interscience, 2dEd 1981). High Electron Mobility Transistors (HEMTs) are another type ofquantum well device and typically use only one half of a quantum well (asingle heterojunction) but may include a stack of a few quantum wells.The HEMT properties arise from conduction parallel to theheterojunctions and in the quantum well conduction or valence subbands;the conduction carriers (electrons or holes) are isolated from theirdonors or acceptors and this isolation limits impurity scattering of thecarriers. See, for example, T. Drummond et al. Electron Mobility inSingle and Multiple Period Modulation-Doped (Al,Ga)As/GaAsHeterostructures, 53 J. Appl. Phys. 1023 (1982). Superlattices consistof many quantum wells so tightly coupled that the individual wells arenot distinguishable, but rather the wells become analogous to atoms in alattice. Consequently, superlattices behave more like new types ofmaterials than as groups of coupled quantum wells; see, generally, L.Esaki et al, Superfine Structure of Semiconductors Grown byMolecular-Beam Epitaxy, CRC Critical Reviews in Solid State Sciences 195(April 1976). Chemla et al, U.S. Pat. No. 4,525,687 and T. Wood et al,High-Speed Optical Modulation with GaAs/GaAlAs Quantum Wells in a p-i-nDiode Structure, 44 Appl. Phys. Lett. 16 (1984) describe a multiplequantum well device for light modulation: an applied electric fieldperturbs the exciton photon absorption resonances near the fundamentaledge of direct gap semiconductors and provides the modulation; the useof quantum wells confines carriers and enhances the exciton bindingenergy. Further, the applied field modifies the envelope wave functionsof carriers in the quantum wells and thus the confinement energies andthe exciton binding energy. The net effect of the quantum wells is apronounced absorption by exciton resonances, and these resonances haveenergies which are easily modifiable by an applied electric field.However, such resonance is extremely sharp and it is a problem tomodulate a fairly broad band of light.

Resonant tunneling devices are the simplest quantum well devices thatexhibit quantum confinement and coupling and were first investigated byL. Chang et al. 24 Appl. Phys. Lett. 593 (1974), who observed weakstructure in the current-voltage characteristics of resonant tunnelingdiodes at low temperatures. More recently, Sollner et al. 43 Appl. Phys.Lett. 588 (1983), have observed large negative differential resistancein such devices (peak-to-valley ratios as large as six to one have beenobtained), and Shewchuk et al, 46 Appl. Phys. Lett. 508 (1985) and M.Reed, to appear, have demonstrated room temperature resonant tunneling.However, resonant tunneling devices have little optical application andit is a problem to apply resonant tunneling to optical devices.

SUMMARY OF THE INVENTION

The present invention provides a light modulator that can be switchedfrom optically reflecting to optically absorbing and devices such asspatial light modulators including an array of such modulators. Thelight modulator relies on tunneling injection and withdrawal of carriersin semiconductors to vary the absorption of incident light. Themodulator may include a stack of quarter wavelength plates ofalternating first and second materials, and in preferred embodimentseach plate of the first material is itself a multilayer stack of quantumwells and tunneling barriers coupling the wells or superlattices.Optical switchability arises from injecting and withdrawing carriers toand from the wells: absorption requires excitation of carriers from thewell levels or minibands to quasi-continuum levels or other minibandsand provides a somewhat broadband modulation. Optionally, both the firstand second material plates are made of multilayer stacks of quantumwells or superlattices. These modulators solve the problems of variationof optical constants and the sharpness of response.

In other preferred embodiments each first material plate is a heavilydoped n⁺ type layer and each second material plate is a heavily doped p⁺type layer so the stack of quarter wavelength plates is also a stack ofp⁺ -n⁺ junctions. Switchability arises from bias variation of thejunctions varying the depletion layer width and consequently theabsorption of photons with energies near the bandgap energy. This leadsto somewhat sharp photon energy response.

Further preferred embodiments use a single thin layer of a few quantumwells for carrier injection and withdrawal and this thin layer isseparated from a mirror by a quarter wavelength plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C schematically illustrate stacked mirrors;

FIG. 2 is a schematic exploded view of a first preferred embodimentstacked modulator;

FIGS. 3A-B are a schematic perspective and conduction band edge views ofa layer of the first preferred embodiment;

FIGS. 4A-B schematically illustrate transmission and absorption by alayer of the first preferred embodiment;

FIGS. 5A-C schematically illustrate injection and withdrawal of carriersto a layer of the first preferred embodiment;

FIG. 6 is a schematic exploded perspective view of a second preferredembodiment;

FIG. 7 is a conduction band edge diagram of a layer junction in thesecond preferred embodiment;

FIGS. 8A-C schematically illustrate injection and withdrawal of carriersto a layer of the second preferred embodiment;

FIGS. 9A-C are schematic perspective view and band diagrams of a thirdpreferred embodiment;

FIGS. 10A-C illustrate in plan and perspective view lateral confinementin the preferred embodiments;

FIGS. 11A-B are schematic plan and cross sectional elevation views of apreferred embodiment spatial light modulator;

FIGS. 12A-C are schematic perspective view and band diagrams for a fifthpreferred embodiment;

FIG. 13 is a schematic perspective view of a sixth preferred embodiment;

FIG. 14 is a graph of the dependence of reflectance upon layer thicknessfor the sixth preferred embodiment;

FIG. 15 is a schematic perspective view of a seventh preferredembodiment; and

FIG. 16 is a schematic prespective view of an eighth perferredembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first preferred embodiment stacked light modulator, generally denotedby reference numeral 30, is illustrated in schematic perspectiveexploded view in FIG. 2 and includes a stack of three layers 32 ofactive material, three layers 34 of transparent material, andsurrounding material and electrodes, not shown for clarity. Activematerial 32 itself is composed of multiple layers of semiconductormaterial as suggested in the exploded portion of FIG. 2. The details ofoperation of modulator 30 are best understood after consideration of theproperties of active material 32, which is itself a multilayer structureas suggested by the upper portion of FIG. 2.

First consider a single crystal of alternating layers of undoped Al_(x)Ga_(1-x) As and undoped Al_(y) Ga_(1-y) As with the layers all about 100Å thick (the height and width may be in the order of a few microns inapplications and do not significantly affect the optical operation) asillustrated schematically in FIG. 3A wherein the layers are labelled bytheir composition; either x or y. The conduction band edge along lineB-B through the alternating layers is schematically illustrated in FIG.3B for x=0.0,y=0.4; note the x layers form quantum wells with barrierheights (the conduction band discontinuity at the interfaces of the xand y layers) of about 0.36 eV (360 meV). Note that these numbers arederived by using the generally accepted partition of the bandgapdifference between AlGaAs alloys into 60% appearing as a conduction banddiscontinuity and 40% as a valence band discontinuity. As will beapparent from the following discussion, the partition has no particulareffect on the operation of modulator 30 beyond adjusting the numbers.

The operation of modulator 30 depends on transition rates of electronsbetween levels in the conduction band wells and tunneling rates, andthus some approximate quantitative analysis will be used as anexplanation aid. However, the approximations used and the analysisperformed should not be construed to be part of modulator 30. Inparticular, we shall use an approximation for an electron in theconduction ban for a direct current bias field by utilizing permanentmagnets to provide the required biasing of the magnetostrictiveelements. Features of the invention include the reduction of coilwinding losses, reduction of wiring complexity and the elimination ofcoupling components which isolate the AC drive from the DC driveresulting in significant simplification of the power driver design.

SUMMARY OF THE INVENTION

The aforementioned problems of the prior art are overcome with otherobjects and advantages of permanent magnet biasing of magnetostrictivetransducers which are provided by magnetic circuitry in accordance withthe invention and utilizes permanent magnets which are magnetized tomuch higher pole strengths that are almost immune to depolarization byalternating flux fields. Samarium-cobalt magnets have these properties.In addition, the shape and relative orientation of the magnets determinethe amount of polarizing flux density that may be uniformly distributedthroughout the magnetostrictive bar. The cross-sectional area of themagnet ends is preferably the same as the cross-sectional area of endsof the bar so that the stray flux density is kept to a minimum therebymaximizing the uniformity of the flux density within themagnetostrictive bar. The magnets are mounted outside the coil that isused for alternating current energization of the magnetostrictive bar tominimize coupling coefficient losses from eddy currents and inductanceleakage which would otherwise be present in greater amounts in themagnets if they were inside the coil.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned aspects and other features, objects, and advantagesof the apparatus of the present invention will be apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein:

FIG. 1 is an isometric view of a preferred embodiment of themagnetostrictive transducer of this invention;

FIG. 2 is a top view of another embodiment of the magnetostrictivetransducer of this invention with biasing magnets on the interiorportion of the transducer; and

FIG. 3 shows a different form of permanent magnet assembly on theinterior portion of the magnetostrictive bars.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an isometric view in partial cross-section and in partialexploded view of a preferred embodiment of a transducer 10 of thisinvention. The transducer 10 comprises radiating masses 11,magnetostrictive bars 12, permanent magnets 13, electrical coils 14, andstress wires 15. The magnetostrictive bars 12 are typically lengthwisegrain oriented bars of the lanthanide series of materials of whichTerfenol (Tb₀.3 Dy₀.7 Fe₂) is preferred. Each bar is electricallyisolated by insulators 12' from the adjacent bar 12 of the stack of bars12' in order to reduce the eddy current losses. Each stack of bars 12'has its ends in contact with the corner blocks 16 so that the assemblyof the stacks 12' and the corner blocks 16 forms a square. Each stack ofbars 12' has an electrical coil or solenoid 14 surrounding it so thatalternating current electrical energization of each coil produces analternating driving field in each stack. The DC biasing flux density foreach stack of bars 12' is provided by a magnet 13. Each magnet 13 isadjacent to and outside each coil 14 surrounding each stack of bars 12'which is to be provided with the DC bias magnetic field. The magnetshave the property that they can be magnetized to high pole strengths andare almost immune to depolarization by alternating flux fields.Samarium-cobalt magnets have been found to be very satisfactory forproducing the DC biasing magnetic flux required by the Terfenol rods 12.These magnets have recoil permeabilities close to that of air as do theTerfenol rods 12. Because of the low permeability of the rods 12, themagnets 13 have like-polarization ends adjacent to each other. The fluxfrom the like-polarity ends of each magnet 13 oppose one another toassist in producing a return flux field on the exterior of the magnet. Aportion of the exterior flux of each magnet passes through and along thelength of the stack of magnetostrictive bars 12' to the other end ofeach magnet where the flux path is completed through the magnet. Thecorner blocks 16 are fabricated from a nonmagnetic material, e.g.,stainless steel. The length and height of the magnet 13 is preferablythe same as the length and height of the stack of bars 12'. The curvedface 13" of magnet 13 has been found to produce a more uniform fieldalong the length of the stack 12' than other configurations. The curvedsurface 13" is preferably a portion of an elliptical surface. Thesurface 13'" of magnet 13 is flat and, as stated previously, adjacent tothe electrical coil 14. It has been experimentally determined for amagnet configuration such as that shown in FIG. 1 that the magnetic fluxdensity at the ends of the bars 12 of stack 12' is about 50 percentgreater than the magnetic flux density at the center of the bar.Optimally, the flux density should be constant throughout each bar 12. Anon-constant flux density moves the operating point for each portion ofthe bar along the B-H curve for the magnetostrictive bar therebyreducing the maximum alternating current field (and hence the acousticpower output) which may be applied before saturation occurs. The lengthof the magnets 13 is preferably equal to the length of each of the bars12 of a stack 12' to obtain a most uniform longitudinal distribution offlux density throughout the bars 12 of stacks 12'.

The magnets 13 are placed outside the coils 14 in order to reduce theeddy current losses in the magnet 13 produced by the AC field of thecoils 14. The radiating masses 11 are attached to corner blocks 16 byscrews 11' which are threadedly engaged with holes 16' in the cornerblocks 16. The radiating masses 11 each have outer surfaces 11" whichform a quarter of a cylindrical surface so that when all four of saidradiating masses 11 are attached to their respective corner blocks 16the resulting transducer has a cylindrical form. Each radiating mass 11is elastically connected to a neighboring mass 11 by a spring 17 whichspans the gap 18 between the masses 11. The portion of the gap 18between spring 17 and the exterior surface 11" is filled with a waterseal 19, typically a urethane, which together with a water proof top andbottom flexible cover (not shown) attached to the radiating masses 11provides a transducer 10 which has a water-proof interior. The covers(not shown) have provision for a cable for supporting the transducer 10and also for providing electrical access to the interior of thetransducer 10. Stress wires 15 are attached by screws 15' between thetops (and bottoms) of adjacent radiating masses 11 and parallel to thestacks of bars 12' to provide compressive stress on the bars 12 and toform the assembly of the transducer 10. The need for compressive stresson the magnetostrictive bars 12 is well known to those skilled in theart, and the details of the use of stress wires 15 to provide thiscompressive stress is described in detail in U.S. Pat. No. 4,438,509incorporated herein by reference and made a part hereof. As described inthat patent, the tensioning of the stress wire 15 by rotatably attachedscrews 15' threaded into the radiating masses 11 causes a compressiveforce on the bars 12 of each stack. The radiating masses 11 aretypically of a nonmagnetic material such as aluminum which has theadvantage of also being of low mass. The magnets 13 exert a repulsionforce on each other and are forced against and held in place by theinner surface 11'" of the radiating means 11.

In operation, the transducer 10 has an alternating voltage applied toeach of the coils 14. For unipolar operation of the transducer 10, i.e.,where the radiating masses 11 move radially in phase with one another,the electrical coils 14 must be energized so that the AC magnetic fluxdirection is in phase for each stack of bars 12' relative to the DC fluxdirection produced by magnets 13 in each stack of bars 12'. Operation ofthe transducer 10 of FIG. 1 using permanent magnet DC flux biasing isslightly less efficient than that obtained when a direct current throughthe coil 14 is used to obtain optimum biasing because of the lessuniform DC magnetic field produced by the magnets 13.

FIG. 2 is a top view of another preferred embodiment of a transducer 20with permanent magnet biasing of the magnetostrictive bars 12. Thetransducer 20 of FIG. 2 is similar to that transducer 10 of FIG. 1 andthe same numbers are utilized as in FIG. 1 to show corresponding partsof the transducer. The transducer 20 of FIG. 2 has, in addition to theelements shown in FIG. 1, a set of inner permanent magnets 22 of thesame samarium-cobalt type as used in the transducer of FIG. 1. However,the magnets 22 are placed on the interior portion of the transducerwithin a nonmagnetic container 23 having at least four opposed walls23'. Typically, the container is of stainless steel. The container isslightly smaller than the inside perimeter formed by the electricalcoils 14, but large enough to contain the magnets 22. Although themagnets 22 are shown in FIG. 2 as touching one another and spaced fromthe container 23, in actuality because of the opposite polarization ofadjacent magnets 22, they will repell one another and be forced by therepulsion force to press against the sides of the container 23. Magnets13, 22 on opposite sides of the same stack of bars 12' havelike-polarity ends adjacent to each other.

It is noted that geometrical constraints on the innermost magnets 22require that they be shorter than the magnetostrictive bars 12. Inasmuchas the magnetic flux 24 produced by the outer magnets 13 produce greaterflux density at the ends than at the center of the magnetostrictive bars12, the shorter length of the inner magnets 22 helps to provide greateruniformity of flux density within the magnetostrictive bars 12 becausethe flux produced by the shorter magnets 22 will be greater near thecenter of the bars than at their extremities. Because eachmagnetostrictive bar 12 is under the influence of the magnetic fieldprovided by the inner magnet 22 and the outer magnet 13, the magneticflux of at least the inner magnets 22 may be reduced to provide a moreuniform flux density in the magnetostrictive bar 12 which isapproximately half of the saturation flux density of each bar 12. Thelesser flux density from each magnet may also be accomplished byreducing the area of the ends 13' and 22' of the magnets 13, 22,respectively. Alternatively, the strength to which the permanent magnets13, 22 are magnetized may be reduced and may differ in order to producea greater uniformity of flux density along the length of themagnetostrictive bar 12. It is noted that, the inner magnets 22 alsohave their innermost faces 22" of eliptical shape with the face 22'"next to coil 14 being flat. The magnets 13 and 22 have the ellipticalsurface only in the circumferential direction.

As noted earlier, the radiating masses 11, the permanent magnets 13 andthe corner blocks 16 are in contact with one another when the screws11', 15' are tightened to form the transducers 10, 20 of FIGS. 1 and 2,respectively. Even after tightening screws 21, the gap 18 still existsin order to provide space for the changing circumference of theradiating masses 11 when they undergo sinusoidal radial expansion andcontraction under the influence of the alternating current in coils 14.

FIG. 3 shows a top view of another structure 29 for obtaining DCmagnetic biasing of the magnetostrictive rods 12. In FIG. 3, thepermanent magnets 30 are trapezoidal and fit inside the container 23 asdescribed earlier. The magnets are forced into the container 23 withlike-polarity poles adjacent each other. Their mutual repulsion forcecauses them to be forced against the side walls of the container 23 andbe maintained in that position. A typical flux line 31 produced by thetrapezoidal magnets 30 is showh in FIG. 3. The uniformity of fluxdensity in the magnetostrictive bars 12 produced by magnets 30 issufficient to result in satisfactory operation of a transducer madeusing trapezoidal magnets 30 without the external magnets 13 of FIGS. 1and 2. Greater uniformity of flux density in the magnetostrictive bars12 of FIG. 3 may be obtained by adding permanent magnets 13 to theexterior surfaces of the coils 14, if desired.

Having described a preferred embodiment of the invention, it will now beapparent to one of skill in the art that other embodiments incorporatingits concept may be used. For example, different shapes of permanentmagnets may provide more uniform fields in the magnetostrictive bars. Inaddition, the invention may be applied to bias magnetostrictive bars in"Tonpilz" and other types of transducers which do not have thecylindrical form used in illustrating the preferred embodiments. It isfelt, therefore, that this invention should not be limited to thedisclosed embodiment, but rather should be limited only by the spiritand scope of the appended claims.

What is claimed is:
 1. A transducer comprising:a paramagneticmagnetostrictive material; a coil for providing an alternating currentmagnetomotive force to said material; permanent magnet means providing amagnetic flux density within and along the length of said material; saidmagnetic flux density within said material provided by the shape of saidpermanent magnet means being substantially uniform over the length ofsaid material; said coil being between said magnetostrictive materialand said magnet means; said magnet means being smaller in transversearea at the ends of said magnet means than at its center and said magnetmeans being uniformly transversely spaced from said coil along thelength of said magnet means; and a mass connected to saidmagnetostrictive material to produce acoustic energy when said coil isenergized with an alternating current to produce said alternatingcurrent magnetomotive force.
 2. The transducer of claim 1 wherein:saidpermanent magnet means is comprised of samarium-cobalt material.
 3. Thetransducer of claim 1 wherein:said permanent magnet means comprises amagnet having a length dimension in the same direction as saidmagnetostrictive material; and said magnet being plano-convex with theflat surface adjacent said coil and the convex surface being curvedalong its length dimension and in the direction of its magnetic field.4. The transducer of claim 3, wherein said convex surface is a portionof an elliptical surface.
 5. The transducer of claim 1 wherein:saidpermamanent magnet means is a bar magnet having oppositely polarizedends; said magnetostrictive material being of substantially the samelength as said bar magnet and having ends separated from the ends ofsaid bar magnet by said coil.
 6. The transducer of claim 1 wherein:saidmagnetostrictive material is comprised of materials from the lanthanideseries.
 7. The transducer of claim 3 wherein:said magnetostrictivematerial is of the composition Tb₀.3 Dy₀.7 Fe₂.
 8. The transducer ofclaim 1 wherein:said permanent magnet means is a plurality oflongitudinal bar magnets each having oppositely polarized ends; and saidbar magnets being on different sides of said magnetostrictive materialwith like polarity poles of saids magnets being in proximity to andnearest to one end of said magnetostrictive material.
 9. The transducerof claim 8 wherein:said magnets are on opposite sides of saidmagnetostrictive material and one of said opposite side magnets isshorter than the other magnet.
 10. A transducer comprising:a firstplurality of lanthanide series material composition magnetostrictivebars; a plurality of coils each providing an alternating currentmagnetomotive force to each of said bars, said bars having two ends,each bar end being adjacent to an end of a different bar; a firstplurality of permanent magnets each having two ends of oppositepolarity; each of said bars having ends in proximity to the ends of atleast one of said plurality of magnets; each of said coils surrounding adifferent one of said bars and being between said bar and one of saidmagnets; and the polarity of adjacent magnet ends being of the samepolarity.
 11. The transducer of claim 10 wherein:said first plurality ofbars comprises a second plurality of bars within each of said coils;said bars of said second plurality being electrically insulated fromeach other.
 12. The transducer of claim 10 comprising in addition:asecond plurality of magnets; each magnet of said second plurality beingon the opposite side of each of said coils from that of the magnets ofsaid first plurality and having the same polarity of magnetizationrelative to the magnetostrictive bar within said coil.
 13. A transducercomprising:a plurality of paramagnetic magnetostrictive bars and aplurality of corner blocks arranged to form a square; said blocksforming the corners of said square of which said bars form the sides; aplurality of coils, one of said coils around at least one bar of saidplurality of bars forming each of said sides; a plurality of permanentmagnets each having opposite magnetic polarization at its ends; each ofsaid magnets being adjacent a respective one of said coils and withmagnet ends adjacent to one of said corner blocks being of likepolarity; a plurality of radiating masses, each mass of said pluralitybeing secured to its respective one of said corner blocks to form acylindrical outer surface; a plurality of stress wires connected betweenthe tops and bottoms of adjacent radiating masses of said plurality toprovide a compressive stress on said magnetostrictive bars; wherebyenergization of said coils with alternating current causes altenatingradial movement of the cylindrical outer surface.
 14. The transducer ofclaim 13 comprising in addition:a square container having four sides andcorners; at least some of said plurality of magnets being within saidcontainer with each corner of said container having magnet ends of thesame polarity, said magnets within said container being repulsed by oneanother to press outwardly upon the walls of said container; saidcontainer being within said plurality of coils.
 15. The transducer ofclaim 15 wherein said container is made of a paramagnetic material. 16.The transducer of claim 15 wherein:each of said magnets within saidcontainer have ends which are bevelled at an angle of forty-five degreesto thereby cause abutting magnets to fill the corner of said squarecontainer.
 17. The transducer of claim 15 comprising in addition:theremainder of said plurality of magnets being on the opposite side ofsaid coils from the sides adjacent said container walls, adjacent endsof said remainder of said plurality of magnets being of the samepolarity.
 18. The transducer of claim 17 in which:each of said coils arewound around a second plurality of bars, each of said second pluralityof bars having ends of like polarity adjacent each other; said bars ofsaid second plurality being electrically insulated from each other. 19.The transducer of claim 15 wherein:said magnets of said plurality withinsaid container having ends which form a 45° angle with respect to thewalls of said container so that each magnet extends to the corner ofsaid container.
 20. The transducer of claim 19 wherein:said remainer ofsaid plurality of magnets have a length substantially equal to thelength of said magnetostrictive bars.
 21. The transducer of claim 19wherein:said remainder of said plurality of magnets have ends each withan area substantially equal to the area of the ends of said bars withineach of said coils.